The determination of protein, DNA and RNA crystal structure using X-ray diffraction methods is an important and expanding area of biology and biomedical science. Many laboratories around the world are involved in X-ray crystallography. The scientific effort in this area is increasing exponentially and as a result of the human genome project, is expected to even further accelerate.
A critical and usually rate-limiting step in the determination of macromolecular structures by X-ray crystallography is the preparation of heavy-atom derivatives of the crystal. Typical derivatives use mercury or lead-based chemicals which bind to the macromolecular crystal. The scientist, however, may need to screen hundreds of such chemicals before finding a successful derivative.
Xenon is a noble gas which binds to specific sites in a macromolecule. Recent experiments have shown that Xenon-protein complexes can serve as heavy atom derivatives in approximately 50% of all cases studied. The Xenon-protein derivative is obtained by equilibrating the protein crystal under a Xenon gas atmosphere, at a relatively low pressure. Unless the pressure is maintained or the complex is frozen to a very low temperature, the Xenon is released from the complex.
Coincidentally, the use of very low temperatures during the collection of X-ray diffraction data is advantageous because it prevents radiation damage to the crystal and affords a more stable crystal, resulting in better data. Collection of X-ray diffraction data at cryogenic temperatures has become the default method for macromolecular crystallography.
Accordingly, there is a need for apparatus which enables incorporation of Xenon atoms into macromolecular structures to enable improved X-ray crystallography thereof. Further, such apparatus must be constructed so as to maintain the macromolecular structure under the influence of the Xenon atmosphere, both during equilibration and during freezing.